This invention is concerned with automatic control of heating and cooling available from a central heating and cooling system as required to meet the maximum heating and cooling demands from a plurality of temperature controlled zones.
In heating and cooling a plurality of rooms or other zones from a central heating and cooling source, some zones have different requirements than others. The heating and cooling demands in zones along an exposed wall of a building will vary with outside air temperature, the position of the sun, direction and velocity of wind, the window area, the insulating properties of the wall, the use of the space comprising the zone, and the personal preferences of the occupants. Interior zones are insulated by the exterior zones from the effects of outside temperature, sun, wind, window area, and wall insulation. They usually demand only cooling to remove heat generated by lights, persons and other heat producers. Exterior zones may require either heating or cooling dependent upon prevailing conditions. As a result of these differing requirements, it has been found necessary, when a central heating and cooling system is employed, to make both heating and cooling available to the individual zones at the same time.
A preferred method for modulating the temperature in the zones involves making both heating and cooling media available to each zone and mixing the media in proportions determined by the temperature deviation from the setpoint in the zone, the flow of mixed media being held substantially constant. In some systems the temperatures of the heating and cooling media has been controlled by the zones having the greatest demand for heating and cooling respectively. In the past, after the zone with the greatest demand for heating had been determined, a heating media control system regulated the temperature of the heating media to produce the required correction in that zone when only heating media was being delivered thereto. An entirely separate cooling media control system operated in similar manner to meet the requirements of the zone with the greatest cooling demand.